22 Use of Alkane Monolayer Templates to Modify The

22 Chapter 2 USE OF ALKANE MONOLAYER TEMPLATES TO MODIFY THE STRUCTURE OF ALKYL ETHER MONOLAYERS ON HIGHLY ORDERED PYROLYTIC GRAPHITE 2.1 Overview Scanning tunneling microscopy (STM) has been used to investigate the structure of pure and mixed monolayers formed by adsorption of long-chain alkanes and/or ethers on highly ordered pyrolytic graphite. Application of a pure phenyloctane solution of simple alkanes, such as tritriacontane, CH3(CH2)31CH3, produced a monolayer within which the individual molecular axes were oriented perpendicular to the lamellar axes. In contrast, a pure solution of symmetrical long-chain ethers, such as di-n-hexadecylether, CH3(CH2)15O(CH2)15CH3, produced a monolayer within which the molecular axes were oriented at an angle of ≈ 65° relative to the lamellar axes. The compositions of the overlying solutions were then gradually changed either from pure alkanes to nearly pure ethers, or from pure ethers to nearly pure alkanes. When ethers replaced alkanes in the monolayer, the ethers conformed to the orientation within the existing alkane layer, rather than adopting the characteristic orientation of pure ether monolayers. However, when alkanes were incorporated into monolayers that had been formed from pure ether solutions, the orientation of the molecules within the monolayer converted to that characteristic of pure alkanes. Alkane monolayers thus acted as templates for subsequent ether layers, but ether monolayers did not act as templates for alkane layers. 2.2 Introduction Long-chain alkanes in solution spontaneously form stable, ordered monolayers on a 1 number of surfaces, including graphite, MoS2, and WS2. This phenomenon is relevant to the fields of separation, adhesion, lubrication, catalysis, and corrosion-resistance. The 23 alkane monolayers can be observed using a scanning tunneling microscope (STM) when an atomically flat surface, such as highly ordered pyrolytic graphite (HOPG), is used as a substrate.2–4 STM observations have routinely shown the formation of single, ordered, alkane monolayer domains > 10,000 nm2 in area. Because alkane monolayers can easily cover surfaces in a highly ordered manner, such monolayers are potentially useful for nanometer-scale surface patterning and for the production of novel materials. The formation of these monolayers is driven by favorable van der Waals and hydrogen bonding interactions between adsorbed molecules, rather than by surface–adsorbate interactions. As a result, the organization of molecules within these layers is typically determined by the structure of the molecules as well as by their functional groups.5–8 The inability to control the manner in which a given molecule will orient within a monolayer has led to the development of two auxiliary methods for generating more intricate monolayer patterns: the use of solutions containing a mixture of molecules, and of molecules with elaborate or chiral structures.9–15 The question of interest in this work was whether the structure of the overlayer formed by a given molecule could be influenced, and in fact templated, by the deliberate prior formation of a monolayer having a different structure. Such behavior would enable manipulation of the structure of the resulting overlayers by prior chemical templating of the surface with monolayers having a structure of interest, constituting a form of overlayer lithography. Since the structures of alkane and ether monolayers are determined by low-energy van der Waals interactions, a monolayer structural template that causes only a limited change to these interactions would produce a monolayer close in energy to that of the original structure. For a templating process to be observable, the structure of the overlayer formed using a template must be close in energy to the structure formed without a template, and one of the two structures must be metastable. We report herein the results of a series of experiments in which we have examined the structures of monolayers produced by adsorption of molecules onto a pristine substrate and the structures formed by substitution of the molecules into a preexisting monolayer having 24 a distinctly different overlayer structure. Specifically, straight-chain alkanes and ethers have been investigated because they are known to form differently structured lamellate monolayers on HOPG which are stable and can be imaged by STM for at least several days after their formation. The direct application of a solution of a single alkane, such as tritriacontane, CH3(CH2)31CH3, in phenyloctane produces a monolayer in which the alkanes are in registry and thus have their individual molecular axes oriented perpendicular to the lamellar axes of the monolayer. In contrast, a phenyloctane solution of a symmetrical long- chain ether, such as di-n-hexadecylether, CH3(CH2)15O(CH2)15CH3, produces a monolayer in which each ether molecule is offset from its neighbors.16,17 This offset produces an angle of ≈ 65° between the molecular and lamellar axes.18 Alkane–ether pairs having the same molecular lengths were therefore selected for study. Due to functional group-related differences in tunneling contrast, alkanes and ethers are distinguishable in STM images that exhibit atomic resolution. Thus, the composition of the resulting overlayer could be determined as a function of the relative concentrations of alkanes and ethers in the overlying solution whenever imaging conditions were ideal. In our work, the compositions of the overlying solutions were changed either from pure alkane to nearly pure ether solutions, or from pure ether to nearly pure alkane solutions. The structures of the resulting overlayers were then investigated by in-situ STM experiments. 2.3 Experimental details Experiments were performed with three length-matched (in their all trans- configuration) pairs of alkanes and ethers: nonacosane and di-n-tetradecylether; tritriacontane and di-n- hexadecylether; and heptatriacontane and di-n-octadecylether (all from TCI America, > 95% purity). Table 2.1 lists the full chemical formulas and abbreviations for each of these compounds. Pure solutions of each of the six compounds were prepared using phenyloctane (Acros, 99% pure) as the solvent. The solvent was approximately saturated with solute at room temperature, and the solutions were filtered before use. The concentrations of the final solutions were determined using an HP 6890 gas chromatograph equipped with a flame ionization detector, with 1-bromohexadecane (Aldrich) as an 25 internal standard. The solubilities were determined to be: C29, 23 mM; C33, 4.5 mM; C37, 2.0 mM; E29, 75 mM; E33, 71 mM; and E37, 4.8 mM. Hence, the solubility of the ethers exceeded that of the alkanes, and the solubility decreased with increasing chain length. For each length-matched pair of alkanes and ethers, mixed composition solutions were prepared by mixing volumes of the two component solutions in 80:20, 60:40, 40:60, and 20:80 ratios. STM images were obtained under ambient laboratory conditions using a Digital Instruments (Veeco) Nanoscope III ECSTM controlled by Nanoscope software version 5.12r2. Tips were mechanically cut from 80:20 Pt/Ir wire. A real-time plane-fitting function was applied to the images while scanning. No additional image corrections were used. Each image consisted of 512 sample scan lines. STM images of pure monolayers were obtained under a drop of the appropriate phenyloctane solution that had been placed on a piece of freshly cleaved HOPG. After ≈ 30–45 min, a drop of the length-matched mixed solution having the largest concentration of the species already present on the surface was then added to the cell. For example, after imaging a monolayer formed from a pure C29 solution, 5 μL of an 80:20 C29/E29 mixed solution was added to the cell. STM images of the resulting monolayer were then obtained after ≈ 30–45 min, to allow some time for equilibration of the mixture. The composition of the overlying solution was altered over several steps using the mixed solutions, to ultimately greatly favor the second component of the length-matched mixture. With C29 and E29 for example, after gradually reducing the concentration of C29 in the overlying solution, a few portions of pure E29 solution were added to the overlying solution, to further eliminate C29 from the system. STM images were collected throughout the course of the experiments. Tunneling tips were not changed during experiments, to avoid mechanically disturbing the monolayers. 2.4 Results Figure 2.1 shows representative STM images of monolayers formed from a phenyloctane solution that contained only an alkane or ether. The orientations of molecules in the lamella of the pure monolayers were consistent with expectations. The alkanes were 26 oriented with their molecular axes perpendicular to the lamellar axes, whereas the ethers were oriented with their molecular axes at an angle of ≈ 65° with respect to the lamellar axes (Figure 2.2). The characteristic monolayer structures of alkanes and ethers shown in Figure 1 were observed for each of the alkanes and ethers studied herein. Table 2.2 lists the unit cell dimensions measured in pure monolayers of each molecule. Although some thermal drift was evident during scanning, its effect was minimal and the drift did not affect the interpretation of the images. The molecules having longer chain lengths formed monolayers more readily than those having shorter chain lengths, and alkane monolayers generally formed more readily than monolayers of the identical chain-length ether, consistent with adsorption isotherm data.19,20 The expected functional group STM constant-current tunneling contrast was also observed, with ethers exhibiting a dark contrast region around the C-O-C functionality relative to the alkanes or relative to the alkyl groups of the long-chain ethers.16–18 As alkanes were added to the overlying ether solution and incorporated into an initially pure ether monolayer, the orientation of all of the molecules within the resulting monolayer was observed to change from 65° to 90°. This transition occurred while the mole fraction of ethers was still high (0.90) in the contacting solution.

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